Phagocytosis of apoptotic bodies by hepatic stellate cells induces NADPH oxidase and is associated with liver fibrosis in vivo
✍ Scribed by Shan-Shan Zhan; Joy X. Jiang; Jian Wu; Charles Halsted; Scott L. Friedman; Mark A. Zern; Natalie J. Torok
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 923 KB
- Volume
- 43
- Category
- Article
- ISSN
- 0270-9139
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✦ Synopsis
Hepatic stellate cell activation is a main feature of liver fibrogenesis. We have previously shown that phagocytosis of apoptotic bodies by stellate cells induces procollagen ␣1 (I) and transforming growth factor beta (TGF-) expression in vitro. Here we have further investigated the downstream effects of phagocytosis by studying NADPH oxidase activation and its link to procollagen ␣1 (I) and TGF-1 expression in an immortalized human stellate cell line and in several models of liver fibrosis. Phagocytosis of apoptotic bodies in LX-1 cells significantly increased superoxide production both in the extracellular and intracellular milieus. By confocal microscopy of LX-1 cells, increased intracellular reactive oxygen species (ROS) were detected in the cells with intracellular apoptotic bodies, and immunohistochemistry documented translocation of the NADPH oxidase p47phox subunit to the membrane. NADPH oxidase activation resulted in upregulation of procollagen ␣1 (I); in contrast, TGF-1 expression was independent of NADPH oxidase activation. This was also confirmed by using siRNA to inhibit TGF-1 production. In addition, with EM studies we showed that phagocytosis of apoptotic bodies by stellate cells occurs in vivo. In conclusion, these data provide a mechanistic link between phagocytosis of apoptotic bodies, production of oxidative radicals, and the activation of hepatic stellate cells. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html). (HEPATOLOGY 2006;43:435-443.)
L iver fibrosis is a wound healing process that is elicited by various toxic stimuli. At the center of the fibrogenic process are the hepatic stellate cells (HSC), which are normally quiescent and produce only small amounts of extracellular matrix (ECM) components for the formation of the basement membrane. Exposure of HSC to soluble factors that include reactive oxygen species (ROS) or cytokines from damaged hepatocytes, activated Kupffer cells, or endothelial cells 4 leads to a morphological and functional transition to myofibroblast-like cells. 5 One of the major profibrogenic mediators is transforming growth factor-beta1 (TGF-1); it is upregulated in activated HSC, and participates in multiple phases of HSC activation. The pathways leading to HSC activation are complex and require better elucidation.
Apoptosis or programmed cell death is also a common feature of chronic liver disease; the end-result of ongoing toxic stimuli that result in the death of hepatocytes. Apoptosis results in the generation of apoptotic bodies (AB), which are subsequently cleared by phagocytosis. After engulfing AB, macrophages secrete TGF-. 8 Indeed, we have previously demonstrated that HSC can phagocytose AB in vitro, which triggers a profibrogenic response with upregulation of TGF-1 and procollagen ␣1 (I) expression. This study suggested for the first time that there may be a direct link between liver injury, apoptosis of hepatocytes, phagocytosis of AB, and activation of HSC.